A cracking furnace is a critical equipment of an ethylene plant. The design of the radiant coils of the ethylene cracking furnace is the critical factor for determining the cracking selectivity, increasing the olefin yields in the pyrolysis products and the flexibility for different feedstocks. Improving the structure and arrangement of the radiant coils becomes the most important part for the technological development of tubular cracking furnace. In the recent decades, several arrangements with different structures, including single-row split diameter-varying tube type, mixed-rows split diameter-varying tube type, non-split diameter-varying tube type, single-pass even-diameter tube type, etc., have been presented.
The arrangement manner of the furnace tubes has been developed from the original single-row manner to the double-row manner. With regard to the single-row manner, it would need more floor space for the same manufacturing capacity, but its advantages are that: the temperature distribution circumferentially around the furnace tubes is uniform and there is little obscur phenomenon; with regard to the double-row manner, it can substantively reduce the floor space of the cracking furnace, however, the obscur phenomenon is very serious and thus the temperature distribution circumferentially around the furnace tubes is negatively affected.
Lummus Crest Inc. (US) discloses a furnace arrangement in China Patent CN1067669 having six first-pass tubes and one second-pass tube, the first tubes being connected at their lower ends via a manifold conduit to the second-pass tube. This kind of structure, as having six first-pass tubes and one second-pass tube, when the furnace tubes are subjected to heat and thus expand, the second-pass tube firstly expands downwardly, and the first-pass tubes move downwardly due to the traction of the second-pass tube, where the first-pass tubes farer from the second-pass tube are subjected to less force, and the first-pass tubes nearer to the second-pass tube are subjected to more force; In addition, due to that the upper and lower manifold conduits are connected rigidly, the expansion difference between the second-pass tube and the first-pass tubes can only be adjusted by the balance system arranged at the inlet of the first-pass tubes, and thus the results are that: when the first-pass tubes cannot move together with the second-pass tube, the furnace tubes will be bent
Exxon Chemical Patents Inc. (US) discloses an arrangement in Patent CN1259981, and meanwhile discloses another arrangement in U.S. Pat. No. 6,528,027. The common drawback of the two different arrangements of the radiant tube are that: as the lower part a first-pass tube inclines outwardly but the second-pass tube do not incline in a reverse direction, meanwhile the adjacent first-pass tube inclines toward the other side, the results are that, when the radiant tubes are subjected to the heating, the overall radiant coils cannot be kept in a single-row, and will naturally present a two-row so as to eliminate stresses. The result is that, the heating of the radiant tubes are not uniform and thus there have the temperature difference between the two sides of the radiant tube walls, the temperature of the side adjacent to burners is high and the temperature of the opposite side is low, thus the furnace tubes will bend toward the burners.
Patent EP1146105 discloses a cracking furnace with such a kind of tube arrangement: two-passes radiant coils are vertically disposed in the firebox of the radiant section, the linear tube portions of the first-pass tubes and the second-pass tubes are arranged in a common plane, each of the first-pass straight tubes and the second-pass tubes is connected to a curved tube via a “S” shaped tube, respectively, the “S” shaped tubes of the first-pass tubes and the second-pass tubes are parallel, respectively, the shape of connecting curved tube may be semi-circle, semi-ellipse or semi-oviform, the angle formed by the respective curved tubes with respect to the plane containing the linear tube portions are the same. This kind of tube arrangement overcomes the drawbacks of the aforementioned radiant tube structure. However, as the 2-1 type furnace tube has a “Y” shaped tube at the lower portion of “two-tube section” of the first-pass tube, tube bending would still exist due to that the two tubes connected to the “Y” shaped tube are different with regard to the expansion due to the heating of the radiant tubes.
When reviewing the aforementioned prior art, it can be seen that none of the various conventional radiant tube arrangements can avoid tube deformation or bending and displacement. Further, this kind of deformation or bending will make that the heating of the radiant tube is not uniform, and thus the radiant tubes will be further deformed and displaced. The heat absorptivity is thus limited and the lifespan of the radiant tubes of cracking furnace is shortened.